In pressure cooking using a pressure container, the major factors affecting the cooking are the application pressure and the time of pressure application.
If the application pressure is now considered, the same is required, from the viewpoint of safety, to be set to the minimum pressure necessary for cooking. Accordingly, for cooking various kinds of materials, it is required that the time of pressure application be adjusted and the cooking effected with the adjusted time of pressure application suitable for the particular material.
If, next, the case is considered in which a pressure application is to be effected, it is so arranged that atmospheric pressure within the pressure container, before heating is raised by heating to a set pressure and thereafter the pressure is held at this value by the opening of a pressure set valve.
Referring to FIG. 3, for example, if it is assumed that in the case of cooking beans in an ordinary container, which is not a pressure container, the internal temperature reaches 100.degree. C after 10 minutes and the cooking is completed by heating for about 120 minutes thereafter as shown by curve A. In the case where a pressure container is used, as shown by curve B, the internal temperature reaches 120.degree. C due to pressure application after the same 10 minutes, and the product is heated at 120.degree. C for about 30 minutes thereafter and the cooking is finished at point a. In either of cases A or B, the time taken to raise the temperature to 100.degree. C or 120.degree. C varies depending on the amount of beans within the container or the intensity of the heating. If the time from the start of the cooking to the finish thereof is comparatively long as in case A, the cooking result is not materially changed if the entire cooking time is set by a timer. In case B, however, if the time from the start of the cooking to the finish thereof is set at a constant value by a timer, for instance, a large difference in the cooking result is produced if the time taken for the temperature to rise to 120.degree. C varies. Namely, it takes 40 minutes from the start of cooking to the finish thereof in case B, but if the time from the start to the finish is tentatively set at 40 minutes on a timer, in the case in which the temperature rises to the set temperature in 5 minutes as shown by curve C due to the fact that the material to be cooked is small in amount or the heating intensity is great, the cooking is finished after 35 minutes so that overcooking results. In the case where the temperature reaches 120.degree. C after 20 minutes from the start of cooking as shown by curve D due to the fact that the material is large in amount or the heating intensity is low, the cooking is terminated 20 minutes after reaching 120.degree. C resulting in insufficient cooking. Consequently, the cooking time after the set temperature has been reached is required to be accurate. Thus, as far as the pressure container is concerned, if the entire cooking time is set by a timer, the cooking result will not be constant if the time taken to reach the set temperature varies depending on the amount of the material to be cooked or on the intensity of heating. Accordingly, these disadvantages can be overcome only if the cooking time after the set temperature or pressure is reached is accurately set.
If, the entire cooking time is now considered, the same can be shortened if it is so arranged that the time after the set pressure influencing the cooking result is reached is kept as proportionately long as possible while the time from the start of cooking to reaching the set pressure is made as proportionately short as possible.